Why do we have to have a combustible engine? Until there is a clean way to produce Hydrogen this isn't the answer. Why can't we develop a source of energy that rejuvenates itself. Has anyone done any research on photosynthesis as an energy source?

I think you are thinking about artificial photosynthesis rather than plant-based natural photosynthesis. If this were indeed possible, artificial photosynthesis would go a long way towards solving the world's energy needs. However, from what I've read so far, artificial photosynthesis is still just a twinkle in the eye of the research scientist and a commercially ready artificial photosynthesis process is nowhere in sight.

There are two means of converting hydrogen back into a usable form of energy: fuel cells and combustion engines. So far, fuel cells are still far from being commercially viable due to their high cost and reliability. This leaves us with combustion engines. Not ideal in many respects but cost effective and proven.

Although artificial photosynthesis is a long way away, other means of generating green hydrogen are being developed such as wind and OTEC. I believe the storage and transport of vast amounts of hydrogen are the real challenges.

We already have an efiecient means of transporting and using green or otherwise, energy sources without having to turn to hydrogen.
It's called electricity!
It has been proven to be much more efficient to use electricity directly in a vehicle than to use it to electrolyze hydrogen from water, transport the hydrogen and then use it in a combustion engine or fuel cell. We do need to move away from the combustion process in our use of all types of fuel. But that is years away. Things we can do today we don't! I keep looking to the EV1 as an example, an overplayed example maybe, but a damn good one!

Bit off topic but...
A few weeks ago was at a classic car show, there putting around was an ols late 1800s early 1900s Daimler (???) with a tiny little steam engine'
There was no sign or smell of fossel fuel and the odd puff of steam.
It was so old it had a tiller steering.
This thing puttered along at a reasonable rate on huge cart wheels, designed for mud roads...smaaler wheels better gearing??
Doesnt have accessories like electic windows etc ....(besides not having windows lol) that add weight...

Taking this concept further and appling to energy effiencey.
If they used the Hitler concept of the 'peoples car" (VW beetle) a basic light weight no frills but comfortable 'cummuting car " applied modern technology to structure, running gear, design...
Surely this would produce a extremely econonic, low cost, low energy to produce since it doesnt need the energy to produce all the extra housings for electic window mottors , coils, wiring etc etc?

As many of you are aware, I also run a 1951 Bradford van daily..
It is 2 cylinders horozonally opposed 1000cc 15 hp 3 speed crash gearbox....wooden frame, un lined, just under 625kgs, no water pump (never over heats in very heavy traffic jams ..convection....no demister etc...
I cruises on the highway at 45 to 50 mph, around town at interesections it keeps up with traffic...they take off faster and I catch up at the next interection.
It will tow a trailer to over 1/2 ton.
It runs at nearly 40 mpg...but considering the engine is 1910 design and meant to run on 65/75 octane..imagine what i would do with a bit of tweaking? and better gearing.

BUT todays population's expectations or a communter car/van with the fancy addons...regardless even if the prospective buyer was a real hard core greeney....I doubt if they would buy it.